NATURE GEOSCIENCE | VOL 6 | NOVEMBER 2013 | www.nature.com/naturegeoscience 905 T he sub-continental lithospheric mantle (SCLM; Box 1) formed in the mid–late Archaean eon 1–5 and changed the face of the Earth. By providing stable continental shelves, or cratons, on an Earth foored by oceanic crust, the SCLM forced a signifcant reorganization of plate tectonics. Te regions surrounding the cra- tons became fractured to form peri-cratonic basins, and disconti- nuities in the SCLM acted as physical guides for magma upwelling from the convecting mantle below. Te SCLM provided a durable, buoyant and rigid reservoir for ore-forming elements, and enabled the preservation of the overlying crust and attendant ore deposits. Te role of the SCLM in generating giant ore deposits is strongly debated. Here we review the characteristics of some diamond, plati- num-group elements (PGE), Ni-Cu-(PGE) and (Cu-)Au ore depos- its worldwide, and argue that the history, structure and evolution of the SCLM may be directly relevant to the genesis and localization of some of these deposits. We therefore suggest that mapping of the SCLM structure, age and composition should form a key compo- nent of mineral exploration programmes. The sub-continental lithospheric mantle Te origins of the SCLM are controversial 1,6–9 . A common view 7,8 suggests that SCLM was built up mostly from subducted oceanic slabs that were depleted in many elements, because of shallow melting at ocean ridges and in subduction zones. A newer view 5,9 , however, suggests that the SCLM formed from the residues of very high-degree melting, either in plumes rising from the deep man- tle 1,5 or through high-temperature melting of the ambient upper mantle 4 . Evidence for formation of the SCLM by high-degree melt- ing is provided by observations of the Fe-depleted compositions (FeO < 7 wt%) of many kimberlite-borne, SCLM-derived perido- tites that have no counterparts in modern oceanic mantle 1,9 . Te initial SCLM was highly depleted in magmaphile elements and has been preserved largely due to the buoyancy imparted by its low iron-to-magnesium ratio. Since the SCLM formed, fuids and magmas derived from the convecting mantle have ponded at its base or ascended via weak zones such as craton boundaries and large faults. Tese magmas and fuids have progressively replenished the SCLM (a process known as metasomatic refertilization) in mag- maphile elements, including components of ore deposits, such as Cu and Au. Refertilization may be especially intense where cratons Continental-root control on the genesis of magmatic ore deposits W. L. Grifn 1 *, G. C. Begg 1,2 and Suzanne Y. O’Reilly 1 Giant magma-related ore systems are prime targets for modern mineral exploration, yet it is unclear what controls their formation. The magmas originate in Earth’s convecting mantle. To reach the surface, they must pass through the stagnant sub-continental lithospheric mantle, but the role of this mantle in ore genesis is vigorously debated. In one view, the ascending magmas are already metal-rich and the sub-continental lithospheric mantle acts only as a passive, buoyant raft on which the continental crust — the fnal store for the ore deposits — rides. Here we argue that the sub-continental lithospheric mantle may actually contain ore-forming elements that could be entrained by ascending magmas, and that it therefore plays a signifcant role in the genesis of magmatic ore. Specifcally, we suggest that some types of magma pick up ore-forming components, such as diamonds and gold, and possibly platinum-group elements, during their passage through the mantle lithosphere, and that the three-dimensional structure of the lithosphere helps to focus deposition of the ore. We therefore suggest that models for ore genesis and exploration need to incorporate the entire lithosphere to be efective. were bordered by subduction zones at some stage of their evolution, because subduction zones provide a supply of fuids. Progressive refertilization has changed the concentrations of major elements and trace elements (as well as their isotopic compositions 10,11 ) in the SCLM. Te most common rocks of the deep SCLM (garnet lherzo- lites) are strongly metasomatized, and carry little information that could be used to support a shallow origin for the SCLM. Ongoing studies 12,13 are integrating geophysics with mantle geo- chemistry and geochronology to map the distribution and age of upper-lithospheric domains to depths ≥100 km (Fig. 1). Te results imply that at least 70% of all SCLM worldwide was formed in a short period 3.0 to 3.5 Gyr ago 1,2 (billion years ago; Ga) and has resided beneath the continents ever since. Similarly, Hf-isotope analyses of crustal zircons 14 indicate that >60% of existing continental crust was generated >2.5 Ga, and has been progressively re-melted and reconstituted in later tectonic episodes. Physical and temporal links between crust and mantle 1,5,12 suggest that large-scale mantle melt- ing produced not only the cratonic SCLM but much of the original continental crust too. Ore deposit links with SCLM structure and composition Diamond deposits. Primary diamond deposits provide a compel- ling example of SCLM control on magma emplacement, and hence on the distribution of ore deposits. Te diamonds occur in dykes and pipes of highly alkaline magmas (kimberlites and lamproites) that are generated by low-volume melting near or below the base of the SCLM. Te magmas pick up diamonds from the deep SCLM (>150 km) during their eruption 7 . Blocks of cratonic SCLM can be imaged by seismic tomography and magnetotelluric surveys 15,16 as volumes with high seismic veloc- ity (due to Fe depletion) and high electrical resistivity. On the large scale (Fig. 2a), kimberlites and other low-volume mantle melts are concentrated near the edges of cratonic blocks. High-resolution seismic tomography (Fig. 2b) shows even more striking correlations; most kimberlites cluster on the edges of high-velocity domains in the deep SCLM. Te correlation is easily understood in geochemi- cal and geophysical terms. Diamond formation requires the metaso- matic re-introduction of carbon into the originally depleted SCLM. Te carbon is typically accompanied by elements such as Ca, Al, K, Na and Fe (refs 10,11), producing lower seismic velocities in the 1 ARC Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Macquarie University, NSW 2109, Australia, 2 Minerals Targeting International PL, 17 Prowse St, West Perth, Western Australia 6005, Australia. *e-mail: bill.grifn@mq.edu.au FOCUS | PERSPECTIVE PUBLISHED ONLINE: 13 OCTOBER 2013 | DOI: 10.1038/NGEO1954 © 2013 Macmillan Publishers Limited. All rights reserved